When an excitation light is illuminated onto a measured object, fluorescence is emitted from fluorescent substances contained in the measured object. The intensity of the fluorescence decays exponentially with the elapse of time from the time of illumination of the excitation light. The fluorescence lifetime expresses the decay characteristic of this fluorescence decay curve and this fluorescence lifetime is determined by the type of fluorescent substance.
In recent years, a measurement method called FLIM (Fluorescence Lifetime Imaging Microscopy), which enables reactions within cells to be measured at high precision by the imaging of the distribution of the fluorescence lifetime values of cells stained by a fluorescent protein or fluorescent dye, has been proposed, and this method is described in the paper, “Fluorescence Lifetime Imaging Microscopy (FLIM): Instrumentation and Applications” (Critical Reviews in Analytical Chemistry, 23(5): 369-395 (1992)). With this measurement method, variations in the fluorescence lifetime, for example, of a fluorescent protein for examining whether or not a particular gene is expressed within a cell, a fluorescent dye that enables measurement of the ionic concentration of calcium, etc., or a fluorescent dye that enables measurement of the pH within a cell, are measured to enable the forming of an image of the distribution of gene expression, ion concentration, pH, etc., within a cell.
As such FLIM measurement methods, a method of combining time-correlation counting by a PMT and laser scanning, a time-resolved image measurement method using a gated image intensifier, and a method using a streak camera are known.